Extremely Soft X-ray Flash as the indicator of oﬀ-axis orphan GRBafterglow
Yuji Urata,1 Kuiyun Huang,2 , Ryo Yamazaki,3 Takanori Sakamoto3
12Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan
Department of Mathematics and Science, National Taiwan Normal University, Lin-kou, New Taipei 2449, Taiwan
3
Department of Physics and Mathematics, College of Science and Engineering, Aoyama Gakuin University,
5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi, Kanagawa 252-5258, Japan
E-mail(FS): urata@astro.ncu.edu.tw
Abstract
Toward unification of GRBs and X-ray Flashes (XRFs), we verified the oﬀ-axis jet model and examineda discovery of oﬀ-axis orphan GRB afterglows. The XRF sample was selected on the basis of the following
three factors: (1) a constraint on the lower peak energy of the prompt spectrum, (2) redshift measurements,and (3) multicolor observations of an earlier (or brightening) phase. XRF020903 was the only sample
selected basis of these criteria. An oﬀ-axis jet model with a large observing angle (0.21 rad, which istwice the jet opening half-angle), can naturally describe the achromatic brightening and the prompt X-ray
spectral properties. This result indicates the existence of oﬀ-axis orphan GRB afterglow light curves.
Events with a larger viewing angle (>2 times of the jet opening angle) could be discovered using Subaru
Hyper-Suprime-Cam (HSC), LSST, and SKA.
Key words: Gamma-Ray Bursts — Oﬀ-axis orphan GRB afterglow — X-ray Flash1.

Introduction

GRBs are believed to be caused by jet collimation andultra-relativistic motion. As identical to that of AGN
classification, a unified picture of GRBs and massive stellar explosions is expected. Along to the observing angleof GRB jet, four astronomical phenomena should exist;(A) classical GRBs, (B) X-ray flash (XRFs), (C) oﬀ-axis
orphan GRB afterglows, and (D) supernova. However,no direct, unambiguous, observational evidence exists.
Toward establishment of the unification model, (1) demostration of oﬀ-axis origin of XRFs and (2) discovery
of oﬀ-axis orphan GRB afterglows are required.
XRFs exhibit that X-ray temporal and spectral properties in the prompt phase do not show any diﬀerences
relative to those of GRBs, except for the considerably
lower energy values of the peak of the νFν spectrum inthe observer’s frame. The majority of HETE-2 samples(nine out of 16 XRFs) show a low energy of spectral peakobs
energy Epeak< 20 keV. The number of XRFs detected
by HETE-2 was comparable and relatively larger thanthat of GRBs indicating that XRFs represent a large
portion of the entire GRB population. To explain theaforementioned prompt observational properties, threemodels have been proposed for XRFs: a high redshift
origin; the oﬀ-axis jet model, which is equivalent to theunification scenario of AGN galaxies; and intrinsic properties (e.g., a subenergetic or ineﬃcient fireball), which
may also produce on-axis orphan afterglows.
Oﬀ-axis orphan afterglows occur as a natural prediction of GRB jets and ultra-relativistic motion. However,no direct, unambiguous, observational evidence exists.
We therefore verify the oﬀ-axis jet model and to provide feedback to ongoing and planned optical time domain surveys of Subaru Hyper-Suprime-Cam (HSC) andLSST (Urata et al. 2015).
2. XRF020903
We considered possible XRFs for our study and quickly
realized XRF 020903 was the only event that has (1)either a measurement or an upper bound on the peaksrc
energy of the prompt spectrum Epeak, (2) a measured
redshift, and (3) multicolor afterglow observations adequately cover the early afterglow phase when achromaticbrightening of the afterglow might occur.This was one ofthe XRFs detected by HETE-2 and the first events forwhich an optical afterglow was detected and a spectroscopic redshift (z = 0.251) was determined. The prompt
emission had the lowest intrinsic spectral peak energy
src
Epeakof 3.3+1.8−1.0 keV among all the XRF samples. We
collected data for the XRF 020903 afterglow by using

0.1

0.01

10−30.1

110100Days after the burst

Fig. 1. Multi-frequency light curves of XRF020903 afterglow. The
dotted lines show the oﬀ-axis jet model functions.

Subaru archive data and photometric results obtainedfrom literature. To perform accurate optical photometry by removing contamination from the host galaxy, wealso employed Pan-STARRS1 data.
Figure 1 shows the optical and radio (8.5 GHz) lightcurves. The optical (Rc and Ic) light curves show aclear rapid rebrightening between 0.7 and 0.9 days. The
equivalent rising power law index α (Fν ∝ tα ) are ∼ 2.6
in the Rc and ∼ 2.3 in the Ic band. There is anotherlate-phase rebrightening peaking at ∼ 20 days in the Rband light curve, which was interpreted as the associatedsupernova component. The SED at 0.7 and 0.9 day iswell fitted by the power-law function as f (ν) ∝ ν −β . We
obtained the β at 0.695 day as 1.48 ± 0.06. Similarly,
we obtained a β as 1.43 ± 0.08 at 0.899 day, which isconsistent with the value for 0.695 day. These results
imply that the rebrightening is achromatic.
To perform light curves and SEDs modeling for
XRF020903, we employed the boxfit code. Figure 1
shows the best-fit model functions that describe theachromatic rebrightening, optical temporal evolution,and radio brightness. There are two notable features.
The first is the jet opening half angle θjet of 0.1 rad,which is consistent with those of classical hard GRBs.
The second is the large observing angle (θobs ∼ 0.21 rad),which corresponds to θobs ∼ 2θjet .
src
We verified the observed small values of Epeak
andEiso by adopting a simple model with a top-hat profileof the prompt emission of a relativistic jet. The paramesrc
ters Epeak(θobs ) and Eiso (θobs ) were analytically derived
as functions of θobs , θjet , and γ. By fixing θobs and θjetsrc
as 0.21 and 0.1 rad, respectively, we evaluated γ, Epeak,src
and Eiso observed from the on-axis of the jet (Epeak(0)and Eiso (0)). These expected values were consistent withthose of classical hard GRBs, and the observed small valsrc
ues of Epeak
and Eiso of XRF 020903 could be naturally
explained by the oﬀ-axis jet model.

3. Prospect with Optical Surveys
We expected oﬀ-axis orphan GRB afterglow light curves
at z = 0.5 along with three viewing angles on the basisof the XRF020903 result (Figure 2 top). To detect theselight curves, especially afterglows with a larger viewingangle (θobs > 2θjet ), an 8-m class telescope with widefield imagers, such as the LSST and Subaru/HSC, is required. Oﬀ-axis orphan GRB afterglows up to ∼ 3θjetcan be discovered by performing time-domain surveys
with an 8-m class telescope. Because such optical timedomain surveys also detect numerous other optical transients (e.g. Urata et al. 2012), we presented expectedradio afterglow light curves for the confirmation and determination of burst parameters. Radio light curves canbe monitored using ALMA and JVLA with reasonable
exposure (Figure 2 bottom).
1

Flux density [mJy]

Subaru Rc
Others R
Subaru Ic x 1.3
Others I x 1.3
Subaru z x 2.5
VLA 8.5GHz

0.1

Optical (r)On-axisiPTF
0.01

Θobs = 2Θjet

Pan-STARRS
Θobs = 3Θjet

10−3

HSC（3σ）

10−4

Θobs = 4Θjet
10−5

Θobs = 2ΘjetRadio (8.5GHz)

Θobs = 3ΘjetΘobs = 4Θjet

On-axisFlux density [mJy]

Flux density [mJy]10.10.01VLA（3σ）

10−30.1

110100Time [Days]
Fig. 2. Expected oﬀ-axis GRB afterglow with observing angles of2θjet (red), 3θjet (green), and 4θjet (blue) at typical redshift ofz = 0.5.

This work is partly supported by the Ministry of Science and Technology of Taiwan grants MOST103-2112M-008-021, 104-2112-M-008-011, and 105-2112-M-008013-MY3 (YU).
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